In recent years, the mobile traffic continues to increase due to the spread of smart phones or the like. In view of the above circumstance, it has been examined to increase cell capacity by introducing beamforming or multi-user multiple-input and multiple-output (MIMO).
“Beamforming” is an example of techniques in which a transmission signal is weighted to direct a transmission beam in a specific direction. Also, “multi-user MIMO” is an example of techniques in which signals addressed to a plurality of users are multiplexed and are simultaneously transmitted.
Japanese Laid-open Patent Publication No. 2013-229727, Japanese Laid-open Patent Publication No. 2013-74629, and Japanese Laid-open Patent Publication No. 2009-296598 discuss related art.
M. J. D. Powell, “A view of algorithms for optimization without derivatives,” Cambridge University Technical Report DAMTP 2007, April, 2007, also discusses related art.